1. Field of the Invention
The present invention relates to a terminal for electrically connecting a button battery in a signal transmitter or the like.
2. Description of Related Art
A conventional terminal for electrically connecting a button battery is shown in FIGS. 7A and 7B. The terminal 400 is made of a flat conductive plate and composed of a terminal base 410 and a pair of spring-contacts 420. The terminal base 410 is mounted on a substrate 20 by soldering. As shown in FIG. 7B, a button battery 30 having a minus-electrode flat terminal 31 and a plus-electrode terminal 32 is contained in a signal transmitter or the like. The pair of spring contacts 420 resiliently contacts the flat terminal 31.
A contact pressure between the spring-contacts 420 and the flat terminal 31 has to be set to a level that attains a good electrical connection. If the contact pressure is lower than a minimum pressure for attaining a good electrical contact (referred to as a required minimum contact pressure), a battery power is consumed in a contact resistance at the contacting point. A width W and a thickness t of the spring-contacts 420 are set to attain the required minimum contact pressure. A spring modulus of the spring contact is determined by the width W, the thickness t and a length L2 of the spring-contact 420. Because an amount of deflection of the spring-contact 420 also depends on its dimensional dispersion and other factors, the width W and the thickness t are set to secure the required minimum contact pressure even if the amount of deflection becomes low.
On the other hand, when the deflection amount of the spring-contact 420 becomes too large due to the dimensional dispersion or other factors, there is a possibility that the spring-contact 420 plastically deforms due to an internal stress exceeding a yield stress. The possibility of such plastic deformation increases as the spring modulus becomes high. Accordingly, it is preferable to set the spring modulus to a sufficiently low level by making the length L2 long. However, the length L2 cannot be made sufficiently long in the conventional structure of the spring contact 420 because it extends straight from the terminal base 410. It would be possible to make the spring modulus low by reducing the thickness t and narrowing the width W. In this case, however, the possibility of plastic deformation cannot be avoided because the internal bending stress in the spring-contact 420 becomes too large.
To cope with the afore-mentioned problem, a terminal 401 shown in FIG. 8 has been proposed. The terminal 401 is composed of a terminal base 411 and three curved spring-contacts 421 connected to the terminal base 411. It is possible to make a length L3 of the spring-contact 421 long and to make the spring modulus low by forming the spring contact 421 in a curved shape. However, another problem is involved in this structure. When the spring-contact 421 is bent by pushing the flat terminal 31 of the button battery 30 against the spring-contact 421, the spring-contact 421 deflects in a twisted manner. That is, a portion along line X-Y moves as a lever having a fulcrum at point Y. In other words, a large force is applied to point X due to deflection of the spring-contact 421. Accordingly, the terminal base 411 connected to the substrate by soldering will be peeled off from the substrate during a long time usage.